X-ray sources in most common use are of two kinds, namely radioisotope sources and X-ray tubes.
Radioisotope X-ray sources consist of small encapsulated quantities of radioactive materials which emit X-rays and/or low-energy gamma radiation. Some radioisotope X-ray sources also emit higher-energy radiation. The quantities of radioactive materials used range from fractions of a microgram to several grams. Such sources are of simple construction, compact and do not require external power supplies for their operation. Their output of radiation is highly stable, but decays over a period of time at known rates, according to the half-life of the active isotope. The encapsulation is generally sufficient to ensure that the source is safe under normal conditions of use in the laboratory and in industry.
The isotope used in radioisotope X-ray sources is selected from about 12 radioisotopes which emit X-radiation and/or low-energy gamma radiation substantially free from higher-energy gamma radiation. Hence the choice of radiation energies for such sources is limited. The output of radiation is determined by the mass in the source and the activity per unit mass of the encapsulated material. However, in practice, both of these quantities have upper limits, so that the maximum available output from a radioactive source is also limited.
Radioisotope X-ray sources are generally costly, and are produced by only a few manufacturers.
An X-ray tube is essentially an evacuated enclosure which contains a source of electrons (or cathode) and a target (or anode). The target emits X-rays when it is bombarded by electrons as a result of the voltage difference applied between the source of electrons and the target. The electrons are usually generated by thermionic emission, the cathode being an incandescent filament. However, cold cathodes (also termed "field-emission cathodes") are used for those applications where a low electron current is acceptable or where one or a few brief bursls of high electron current are sufficient (such as in flash X-ray tubes).
The output of radiation from an X-ray tube is determined by factors such as the electron current, the voltage difference between the cathode and the anode, and the atomic number of the material of the target. The energy spectrum of the emitted radiation is also controlled by these factors. In general, the output of an X-ray tube is much higher than the outputs that are available from radioisotope sources (up to several orders of magnitude greater), and the energy spectrum of the emitted radiation can be controlled within wide limits.
However, X-ray tubes are usually bulky pieces of apparatus and even the smallest tubes have a diameter of from 50 to 100 mm and a length of about 200 mm. They require an external power supply and must be cooled to remove the heat generated by the filament and the target.
Another problem associated with X-ray tubes is that they frequently fail after relatively brief periods of use due to some heat-induced cause, such as gas-evolution from the target or evaporation or burn-out of the filament material. Cold cathode X-ray tubes are not prone to thermal damage, but they often exhibit rapid drops in output due to blunting of, erosion of, or other changes to the pointed ends of their field-emitting cathode.